Flip-chip attach processes typically involve a reflow of solder bumps to form solder joints between a die and substrate. The substrate usually includes substrate bumping sites thereon, and the die includes die bumping sites thereon adapted to be joined to the substrate bumping sites to establish an electrical connection between the die and the substrate. Solder bumps are provided onto bumping sites of the substrate and/or die. Typically, temperatures necessary to reflow the solder bumps lead to an expansion of each of the die and the substrate. During cooling, different shrinkage amounts of the die and substrate resulting from a mismatch between the relative coefficients of thermal expansion (CTE's) of the die and the substrate do sometimes lead to cracks within the die and/or to a shear deformation of the interconnect joints, especially when a mechanically weak interlayer dielectric (ILD) is used on the die. The ILD of the die usually tends to experience increased thermo-mechanical stresses in the area under the solder joints during die and substrate attach, which stresses lead to increased under bump ILD cracking.
One method the prior art uses to mitigate the above problem is to use lead based die bumps or lead based substrate solder, which tend to deform easily and to therefore at least partially accommodate the CTE driven deformation between the die and the substrate. However, the negative environmental impacts resulting form the use of lead-based bumps and solder are well known and documented.
Another method the prior art uses to mitigate the above problem is to use underfill materials compensate for the differences in CTE of the die and the substrate before the joint, die, and substrate cool down. For example, capillary underflow regime or a no-flow underfill regime may be used to reduce the effects of a CTE mismatch between die and substrate. However, disadvantageously, even such underfill regimes sometimes cannot always effectively mitigate the problems associated with a die-substrate CTE mismatch.
The prior art fails to provide an effective and reliable method of joining a die to a substrate.
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